Yong Ju Park,†,^ Seoung-Ki Lee,†,^ Min-Seok Kim,‡ Hyunmin Kim,§ and Jong-Hyun Ahn†,* †
School of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, Korea, ‡Center for Mass Related Quantities, Korea Research Institute of Standards and Science, Daejeon 305-340, Korea, and §Nano & Bio Research Division, Daegu Gyeongbuk Institute of Science and Technology, Daegu 711-873, Korea. ^Yong Ju Park and Seoung-Ki Lee contributed equally to this work.
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Graphene-Based Conformal Devices
ABSTRACT Despite recent progress in bendable and stretchable
thin-film transistors using novel designs and materials, the development of conformal devices remains limited by the insufficient flexibility of devices. Here, we demonstrate the fabrication of graphene-based conformal and stretchable devices such as transistor and tactile sensor on a substrate with a convoluted surface by scaling down the device thickness. The 70 nm thick graphene-based conformal devices displayed a much lower bending stiffness than reported previously. The demonstrated devices provided excellent conformal coverage over an uneven animal hide surface without the need for an adhesive. In addition, the ultrathin graphene devices formed on the three-dimensionally curved animal hide exhibited stable electrical characteristics, even under repetitive bending and twisting. The advanced performance and flexibility demonstrated here show promise for the development and adoption of wearable electronics in a wide range of future applications. KEYWORDS: ultrathin transistor . tactile sensor . graphene . conformal device . wearable electronics
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ecently, conformal devices on uneven surface have garnered great popularity due to their multifaceted applications in healthcare monitoring systems13 and wearable electronics.4 Such devices are hardly achievable using a sophisticated planar layout; rather, the fabrication of them strictly requires a new design of device geometry and a relevant adopt of materials that can compensate structural imperfections. Thus, most of the current research has been focusing on how to maximize the structural compliance of “implanted” devices with regards to the environment; strain is regarded as one of the major critical challenges to overcome, the solution of which is not fully provided yet, especially as for the mechanocompatibility of the devices with respect to in vivo circumstances. Strain, which can seriously degrade device performance, is locally generated and distributed across the device in a physical contact with a rough surface. In an effort to address this issue, various approaches have been developed. The representative methods have been used by employing elastic conductors composed of elastic rubbers and conductive materials such as micrometer-sized silver flakes, metal wires and carbon nanotubes.57 Net-shaped and fractured metallic film has PARK ET AL.
also been demonstrated as a solution that can absorb any strain by bending/flexing of the devices.810 Although these approaches are potentially useful in three-dimensional electronics systems, its application is restricted to the objects with simple curvatures. Another approach for conformal devices involves interconnecting a series of rigid active devices with “shock-absorbing” bridges that can accommodate both compressive and tensile stresses.1115 This method has been exemplified in a course of applications, including electronic eye cameras,16 stretchable transistors,17 and flexible solar cells.18 Unfortunately, various additional processes must be accompanied to achieve intended results in this case. Prestraining or premolding substrate can limit the range of available applications. Indeed, the delicate tolerance to the collision and friction stemming from out-of-plane buckling of the interconnector becomes a significant hurdle for this method. Recently, epidermal electronics based on the combination of ultrathin silicon membrane with filamentary serpentine metal mesh have been extensively exploited; some applications of these systems are medical sensors to monitor body temperature3 and neural/electrophysiological signals.2,4,19 Although there are considerable advantages with established silicon VOL. 8
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[email protected]. Received for review June 25, 2014 and accepted July 29, 2014. Published online July 29, 2014 10.1021/nn503446f C 2014 American Chemical Society
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RESULTS AND DISCUSSION Reducing the device thickness is one strategy for realizing conformal devices because the bending stiffness is proportional to the cube of the thickness.25 Considerable efforts have been applied toward thinning the device supporting layer since the supporting layer contributes majority of the device thickness and degrades the flexibility of the system.26 To this end, we designed the fabrication of a stand-alone device prepared without a substrate by substituting a constituent layer that comprises the transistors for the supporting layer. This device structure could be achieved because the graphene layer used as both channel and electrodes was thin, light, and tough, and it did not require an additional supporting layer. PARK ET AL.
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technology in the device design, the siliconmembrane-based devices still have a critical drawback in bending inflexibility even when assembled with the soft substrate because the active layer is several hundred nanometers thick. A smaller bending “stiffness” would be desired to achieve an enough conformal coverage over the uneven “wrinkle-prone” substrate, such as the human skin. Also, the silicon membrane requires an encapsulation layer to obtain conformal attachment to a substrate for stable operation during macroscopic movements of the joints in the human body (i.e., wrist, knee, and elbow), becoming an additional source for the bending stiffness increase.20 In this light, graphene is an ideal constituent for conformal devices because it is a one-atomic-thick platform with extraordinary mechanical flexibility and optical transmittance.21,22 A few recent studies have reported the preparation of graphene- or carbon-based flexible field effect transistors on unconventional substrates;23,24 however, it still remains a significant challenge to integrate the graphene devices on rough substrate such as human skin or animal hide without degradation of electrical characteristics. In this work, we report the graphene-based devices that can be attached conformably to highly deformed surface such as an animal hide and exhibit stable electrical and mechanical performances under serious external deformations, such as flexion and distortion. With direct application of a polymer film used as a gate dielectric and as a supporting layer of device, the thickness of graphene transistor can be reduced less than 70 nm. Consequently, the demonstrated graphenebased conformal device produced a bending stiffness (EI ∼1.24 GPa 3 μm4) that was much lower than the values in the literature, leading to an excellent adaptational coverage over uneven surfaces without necessity of an adhesive layer. Furthermore, we demonstrate a tactile sensor-array composed of monolithically patterned graphene film, showing good sensitivity against external force.
Figure 1. (a) Schematic diagram showing the procedure used to fabricate array of ultrathin graphene field effect transistors (UT-GFETs). (b) Microscopy image of the asfabricated UT-GFETs. The gray regions indicate the gate electrode, and the green regions indicate the channels and source/drain electrodes. (c) Optical image of the UT-GFETs floating on DI water during the transfer process. (d) Optical image of the transferred UT-GFETs on a glass substrate.
Figure 1(ad) shows schematic diagrams of the entire process for fabricating ultrathin graphene field effect transistors (UT-GFETs) and the optical images of device at each step. First, few-layer graphene (FLG) was synthesized on a Ni catalyst using chemical vapor deposition (CVD). The FLG was then transferred onto a SiO2/Si substrate.27 The gate electrode was defined using conventional photolithography and oxygen plasma etching processes, as reported previously.22 After removing the photoresist, SU-8 epoxy, which functioned as both gate dielectric and supporting layer for GFET,28 was spin-coated onto the device, crosslinked by UV irradiation, and hard-baked. The thickness of the epoxy film was tuned from 70 to 1400 nm by using different volumetric ratios of the epoxy and epoxy thinner. Because the graphene thickness was subnanometer in scale, the total thickness of the GFET device could be regulated by controlling the thickness of the SU-8 epoxy layer. The UT-GFETs were completed by patterning the source (S)/drain (D) electrodes and semiconducting channel region, after transferring the single layer graphene (SLG) grown on Cu foil onto an epoxy coated device.29,30 The detailed fabrication step was described in Supporting Information. The fabricated UT-GFETs on the SiO2/Si handling substrate had a channel width of 25 μm and length of 30 μm, as shown in Figure 1(b). The UT-GFETs were then floated on the surface of a dilute HF solution (HF:DI water =1:10) at room temperature to etch away the SiO2 sacrificial layer.31 In this step, the epoxy dielectric film acted as a VOL. 8
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γ g γc0 ¼
EI f1 þ (1 þ λ)R2 =[(1 λ)γ20 ]g 2R2 b
(1)
where γ (10 mJ/m2) is the adhesion energy in the wet 0 state,1,33 γc is the calculated adhesion energy, EI is the PARK ET AL.
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supporting layer to protect the device from destruction during the transfer process. Thus, we avoided additional spin-coating steps that would require conventional transfer steps in an effort to simplify the fabrication process.32 Complete removal of the sacrificial layer released the UT-GFETs from the handling substrate, and the UT-GFETs freely floated on the surface of the etchant. The UT-GFETs were rinsed with DI water to remove remaining etchant and were transferred to the target substrate. The outstanding flexibility of the device due to the extreme thinness of the device may be appreciated from the photographic image of the floated UT-GFET film shown in Figure 1(c). The dry transfer method is also described in the Supporting Information. Finally, the devices were transferred to a rough substrate with the help of the capillary forces of water during drying and adhered to the substrate through van der Waals forces alone. Figure 1(d) shows a photograph and magnified image of the UT-GFETs transferred to transparent substrates. As expected, the optical transmittance exceeded 70% across the visible spectrum (Supporting Information, Figure S3). The transmittance was reduced by ∼3% in the source (S)/drain (D) electrodes and channel region, and by ∼26% at 550 nm in the gate electrode and epoxy dielectric films. In order to achieve full range of conformal contact between a GFET film and a rugged surface, an adhesive energy was investigated using numerical calculation of two overlapping cylinder models, and the results were compared with experimental measurement as shown in Figure 2. Since the adhesive energy depends strongly on the bending stiffness of a device, a bending stiffness was calculated based on the thickness of the device as well as the mechanical properties of each layer (i.e., Young's modulus and the Poisson's ratio). Figure 2(a) shows the calculated bending stiffness (EI) variations of fabricated GFETs as a function of the thickness. The bending stiffness decreased dramatically as the device thickness decreased. At a thickness of 1400 nm, the EI was calculated to be 9192.4 GPa 3 μm4. This value decreases to 266.35, 36.16, and 1.15 GPa 3 μm4 as the GFET becomes thinner to 430, 220, and 70 nm, respectively. Experimentally measured EI as marked by red symbols was well matched with the calculated EI (Supporting Information, Figure S4). The thinnest GFET among the various devices with different thickness had a value of 1.24 GPa 3 μm4. It is worth noting that an EI of 1.24 GPa 3 μm4 for the 70 nm thick UT-GFETs is the lowest value yet reported for such devices (Supporting Information, Table S2). Finally, the adhesion energy could be calculated according to the following equation:1
Figure 2. (a) Calculated stiffness of a UT-GFET prepared with a range of device thickness values. Red square indicate the experimentally measured stiffness values at 70, 220, 430, and 1400 nm, the thicknesses of the fabricated UT-GFET devices. (b) Calculated adhesion energy of the UT-GFET as a function of device thickness. Mechanical modeling revealed that the UT-GFET film could achieve conformal contact in devices
